1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying sheets along a curved guide surface through a pair of conveyance rollers disposed at an upstream side and at a downstream side of the guide surface and an image forming apparatus with the sheet conveying apparatus.
2. Description of the Related Art
In an image forming apparatus such as a printer, a copying machine, a facsimile machine, or the like, a sheet before or after it is provided with an image is conveyed by a sheet conveyance apparatus. Various kinds of sheets are used as a sheet on which an image is provided. For example, art paper having high surface smoothness used in image formation such as a photograph and, to the contrary, recycled paper having low surface smoothness, or the like are used in addition to copying paper in general use. Also, each kind of paper includes different sizes from a larger size to a smaller size, or different thicknesses from a thicker one to a thinner one. Namely, various kinds of sheets in various conditions and sizes are used.
On the other hand, a sheet conveyance path through which a sheet is conveyed is provided with a guide plate for guiding the sheet to be conveyed. Generally, the guide plate is provided with projecting guide ribs extended to a sheet conveyance direction. Many guide ribs are provided almost throughout the width in a sheet passing width direction (a direction orthogonal to the sheet conveyance direction). For example, when an A4 size sheet is conveyed in a portrait orientation, the shorter side of the A4 size sheet is the sheet passing width and the guide ribs are provided almost throughout the sheet passing width, such that increase of conveyance resistance caused by direct contact of the sheet to be conveyed with the a guide plate can be prevented (e.g., Japanese Patent Application Laid-open Publication No. 2000-95382).
In the above-described sheet conveying apparatus, however, there has been a problem that friction between the guide ribs and the sheet to be carried would cause a conveyance friction noise. More specifically, a large conveyance friction noise used to occur when a pair of conveyance rollers is provided at an upstream side and at a downstream side in the sheet conveyance direction and guide ribs curved into a convex shape toward the sheet conveyance path are provided between the pair of conveyance rollers. That is, when the sheet is conveyed such that the trailing edge of the sheet is nipped by the pair of conveyance rollers at the upstream side and the leading edge of the sheet is nipped by the pair of conveyance rollers at the downstream side, the sheet is pulled because of the linear speed of both pairs of conveyance rollers or a drive control timing of both pairs of conveyance rollers and frictionally rubbed against the guide ribs curved into the convex shape with a strong force. Therefore, the conveyance friction noise tends to be louder. This tendency becomes more remarkable when a sheet having lower surface smoothness such as recycled paper is used.
Also, there may be such a case that a leading edge of some kinds of sheets, e.g., a curled sheet, a sheet having one side printed, a sheet having a cutting burr at its edge, and a thick rigid sheet, may cause a friction noise when it hits the guide ribs. In recent image forming apparatuses, many of them have a sheet conveyance path which curves with a small curvature in order to downsize the apparatus, such that a guide plate which is placed in such a sheet conveyance path tends to generate the friction noise. Further, depending on the angle of approach of the leading edge of the sheet to the guide plate, there are such inconveniences that the sheet may be buckled and the leading edge of the sheet may hit projecting side walls of the guide ribs, causing the sheet to skew.